The high cost involved in the mining and processing of fossil fuels, their depleting resources, and the limited global reserves of crude oil has caused greater interest in renewable resources. Most commonly, the fuels obtained from renewable resources are ethanol from maize and sugar, as well as vegetable oils used as diesel fuel. In the area of diesel fuel obtained from renewable sources, there are two sub-areas: bio-diesel (fatty acid methyl esters), described for instance in: D. Srivivas, J. K. Satyarthi, Catal. Surv. Asia, 2011, 15, 145, E. F. Romalho et al., J. Therm. Anal. Calorim., 2011, 106, 825, C. L. Bianchi et al., Catal. Lett., 2010, 134, 179 and green diesel, i.e., green diesel fuel (paraffins obtained from the fatty acids present in vegetable oils and animal fat) which is known, for instance, from the U.S. Pat. No. 8,119,847. Methyl esters, mainly those of linear C14 to C22 carboxylic acids, may be used as fuel directly or mixed with diesel fuel from crude oil refining. The use of fatty acid methyl esters as diesel fuel requires costly modifications of diesel engines and of injectors. The use of biodiesel increases operating costs because of its low lubricity, and winter weather conditions significantly limit its usability.
Taking into consideration the above-mentioned limitations in the use of biodiesel, alternative fuels are being sought. There is a potential for making green diesel (II and III generation biofuels) from renewable raw materials by converting fatty acids from triglycerides and/or free fatty acids originating from natural fat, especially waste vegetable oils, animal fat or algal oils to obtain linear aliphatic saturated hydrocarbons (paraffins). Green diesel has a high cetane number, which is necessary in maintaining the good performance of diesel engines (U.S. Pat. No. 8,119,847) and may be used as an independent fuel or mixed with diesel from crude oil. Its use does not require engine modifications and it can be processed in existing refineries adapted to the refining of crude oil.
Recently, various types of waste materials are regarded as a source of raw materials for making other higher-value products. Vegetable oils, animal fat and various kinds of waste edible fat are part of the waste classified in the Waste Catalogue (Polish Journal of Laws Dz. U No. 112 of 2001, Item 1206), principally, in Group 2. Such waste must be disposed of properly so as not to create any environmental hazard. Algae with high lipid content may be especially valuable (oil content in certain species is as high as above 80% of dry algal biomass) as it may constitute a raw material for third generation biofuels. The use of algae for energy has a huge potential because algae quickly adapt to growth conditions, may be grown both in fresh and sea water, and also because land is not required for production. Furthermore, due to the fact that two-thirds of the earth's surface is covered with water, algae will be a renewable source with a huge potential for the global energy needs.
A higher share of biocomponents in the market for liquid fuels, and liquid biofuels for use in transportation, is an important element of sustainable development, leading to an improved energy security by diversifying fuel supply sources and reducing dependence on petroleum imports while, in addition, having a positive impact on natural environment through appropriate waste management. According to forecasts, there will be a dynamic growth of the market for synthetic hydrocarbons obtained from biomass, as such will provide a substitute for petroleum in the future.
Two methods of obtaining paraffins from fatty acids are described predominantly in the literature: hydrodeoxygenation, HDO (R—COOH+3H2→R—CH3+2H2O), and decarboxylation (R—COOH→R—H+CO2), for example in J-G. Na et al., Catal. Today, 2012, 185, 313. In HDO, oxygen in the form of water is removed from fatty acids (hydrogenolysis). Typical hydrorefining catalysts, such as Ni/Mo or Co/Mo, are used in the HDO process. The HDO method ensures the production of pure hydrocarbons, which are fully compatible with conventional fuels. However the process is energy-consuming because it requires the application of a high-pressure stream of hydrogen. It would be necessary to minimize the use of hydrogen for the process to be commercialized. The process of decarboxylation of fatty acids, with CO2 removal from the molecule, is an alternative to HDO, though the hydrocarbons obtained are of one carbon atom less than in the molecule. In contrast to HDO, water is not produced in the decarboxylation process; this has a favorable effect, among others, on catalyst activity. On the other hand, both methods are conducted in batch systems at elevated pressures.
The U.S. Pat. No. 8,119,847 describes single-stage processing of vegetable oils and animal fat to paraffins by HDO in the presence of active metals and their mixtures (Ni, Co, Mo, W, Ni/Mo, Co/Mo), supported on graphite or oxides of aluminum and silicon, as well as zeolites (ZSM-5, ZSM-11, zeolite Y, mordenite, bata). The process was conducted in an autoclave at temperatures in the range of 250-450° C. and hydrogen pressures in the range of 3.4-17.2 MPa. Paraffins were obtained, with high selectivity, in which the ratios of odd to even-numbered carbon atoms in the hydrocarbon chain were 2:1.
M. Snare et al., Fuel, 2008, 87, 933 reported a method to obtain hydrocarbons from vegetable oils and animal fat over Pd/C at temperatures in the range 300-360° C. and at hydrogen pressures in the range 1.5-2.7 MPa. In addition to hydrocarbons, a significant amount of unreacted carboxylic acids was found in the product.
J-G. Na et al., Catal. Today, 2012, 185, 313 reported an innovative method to obtain paraffins in a decarboxylation reaction of fresh-water microalgae oil. The algal oil containing 36% triglycerides was initially subjected to pre-pyrolysis (600° C., 1 hr) to obtain a product for decarboxylation containing C16 and C18 free fatty acids, hydrocarbons and compounds containing pure heteroatoms of nitrogen or sulfur. Volatile acids, aldehydes, ketones and furane compounds obtained during the pyrolysis process were separated from the stream which was directed for the decarboxylation process. The decarboxylation process was carried out in an autoclave in a temperature range of 350-400° C. in the presence of hydrotalcites. C15 and C17 alkanes and also partly unreacted carboxylic acids were obtained mainly. The diesel fraction content was 35% after the first stage (pyrolysis) and 83.8% after decarboxylation.
Approaches to producing paraffinic hydrocarbons may benefit from improvements.